1. Field of the Invention
The present invention relates to an angular acceleration sensor configured to detect an angular acceleration from flexural stress that is generated in a beam, and also relates to an acceleration sensor configured to detect an acceleration from flexural stress that is generated in a beam.
2. Description of the Related Art
Some type of angular acceleration sensor and acceleration sensor includes a weight portion, a beam, and a detection portion, and detects an angular acceleration or an acceleration, each acting on the weight portion, from flexural stress that is generated in the beam supporting the weight portion (see, e.g., Japanese Unexamined Patent Application Publication No. 08-160066).
An example of general configuration of an angular acceleration sensor will be described below.
FIG. 5A is a plan view illustrating a first related-art configuration example of an angular acceleration sensor. In the following description, it is assumed that an axis extending in a flexing direction of a beam is defined as an X-axis of an orthogonal coordinate system, an axis extending in a lengthwise direction of the beam is defined as a Y-axis of the orthogonal coordinate system, and an axis extending perpendicularly to both the X-axis and the Y-axis is defined as a Z-axis of the orthogonal coordinate system.
An angular acceleration sensor 101 according to the first related-art configuration example includes a stationary portion 102, a weight portion 103, a beam 104, and two piezoresistors 105A and 105B. The stationary portion 102 is fixed to, e.g., a not-illustrated casing that is arranged at a position along a direction of the Z-axis relative to the stationary portion 12. The beam 104 extends along the Y-axis in a state floating from the casing, etc. An end portion of the beam 104 on the positive direction side of the Y-axis is connected to the weight portion 103, and an end portion of the beam 104 on the negative direction side of the Y-axis is connected to the stationary portion 102. The weight portion 103 is held in an X-Y plane at a position spaced from the stationary portion 12 in a state floating from the casing, etc. The piezoresistors 105A and 105B are disposed on the beam 104 to lie in an X-Y plane side by side in the X-axis direction, and they have a rectangular shape with a lengthwise direction thereof extending in the Y-axis.
FIG. 5B is a contour view illustrating a distribution of flexure stress that is generated in the beam 104 when the beam 104 is flexed toward the negative direction side of the X-axis in the angular acceleration sensor 101 according to the first related-art configuration example.
In the angular acceleration sensor 101, when an angular acceleration acts on the weight portion 103 in a clockwise direction as viewed from the positive direction of the Z-axis, the beam 104 is flexed toward the negative direction side of the X-axis. Correspondingly, compression stress acts on a region of the beam 104 near a lateral surface thereof on the negative direction side of the X-axis, and tensile stress acts on a region of the beam 104 near a lateral surface thereof on the positive direction side of the X-axis. A line (denoted by a one-dot-chain line) passing a center of the beam 104 as viewed in a widthwise direction (i.e., in an X-axis direction) defines a boundary between the tensile stress and the compression stress.
FIG. 5C is an illustration to explain a detection circuit included in the angular acceleration sensor 101 according to the first related-art configuration example.
The piezoresistors 105A and 105B are connected in series to a constant voltage source, and they constitute a resistive voltage-dividing circuit. The piezoresistors 105A and 105B are arranged parallel to each other on both sides of the line passing the widthwise center of the beam 104. Therefore, when the beam 104 is flexed in the X-axis direction, the compression stress is caused to act on the piezoresistor that is arranged in a region on one side of a neutral plane of the beam 104, and the tensile stress is caused to act on the piezoresistor that is arranged in a region on the other side. Accordingly, the piezoresistors 105A and 105B are expanded and contracted oppositely to each other. A resistance value of the expanded piezoresistor increases, whereas a resistance value of the contracted piezoresistor decreases. Thus, a voltage division ratio between the piezoresistors 105A and 105B in the resistive voltage-dividing circuit is changed, and a voltage across one of the two piezoresistors corresponds to the angular acceleration acting on the weight portion 103.
FIG. 6A is a plan view illustrating a second related-art configuration example of the angular acceleration sensor.
An angular acceleration sensor 201 according to the second related-art configuration example includes a stationary portion 202, a weight portion 203, a beam 204, and four piezoresistors 205A, 205B, 205C and 205D. The stationary portion 202, the weight portion 203, and the beam 204 have similar structures to those described above in the first related-art configuration example. The piezoresistors 205A, 205B, 205C and 205D are arranged not only symmetrically with respect to a line (denoted by a one-dot-chain line in FIG. 6B) passing a center of the beam 204 as viewed in a widthwise direction (i.e., in an X-axis direction), but also symmetrically with respect to a line (not illustrated) passing a center of the beam 204 as viewed in a lengthwise direction (i.e., in a Y-axis direction).
FIG. 6B is an illustration to explain a detection circuit included in the angular acceleration sensor 201 according to the second related-art configuration example.
The piezoresistors 205A, 205B, 205C and 205D constitute a bridge circuit such that every two piezoresistors arranged symmetrically to each other are connected in series, and two sets of serial circuits are connected to a constant voltage source or a constant current source in parallel. In the bridge circuit thus constituted, potentials at two output terminals are changed in mutually reversed polarities upon flexure of the beam 204 in the X-axis direction, and an angular acceleration can be measured by taking out a potential difference between the two output terminals as a voltage variation.
In any of the above-described related-art configuration examples, a distribution of flexure stress occurs on the beam in a state where the beam is flexed, as illustrated in FIG. 5B. Reviewing here the distribution of flexure stress in the Y-axis direction, i.e., the lengthwise direction of the beam, in detail, the flexure stress increases at a position nearer to the center of the beam in the Y-axis direction, and the flexure stress decreases at a position farther away from the center of the beam in the Y-axis direction. Accordingly, when the piezoresistors are arranged at the center of the beam in the Y-axis direction as in the first related-art configuration example, maximum flexure stress is caused to act on the piezoresistors. However, when the piezoresistors are arranged at positions deviated from the center of the beam in the Y-axis direction as in the second related-art configuration example, just smaller flexure stress than the maximum flexure stress is caused to act on the piezoresistors.
Stated in another way, in the related-art configuration examples, since the flexure stress is maximized at the center of the beam in the lengthwise direction of the beam, the maximum flexure stress cannot be detected by the piezoresistors when the piezoresistors are arranged at positions deviated from the center of the beam in the lengthwise direction of the beam. This leads to a problem that stress generated in the beam cannot be efficiently detected by the piezoresistors, and sensitivity of the angular acceleration sensor is low. Such a problem occurs not only in the angular acceleration sensor, but also in an acceleration sensor which includes a stationary portion, a weight portion, a beam, and a piezoresistor, in a similar way.